1. Field of the Invention
The present invention relates to a screen and an image projection apparatus used for, for example, projection television sets for modulating light that contains coherent light and that is emitted from a light source and projecting the light onto the screen.
2. Description of the Related Art
Image projection apparatuses (optical projectors) are known that illuminate a light modulation unit, such as a liquid crystal panel, a digital micromirror device (DMD), or a light modulation element using a diffraction grating, using an illumination unit so as to project light transmitted or reflected by the light modulation unit onto a screen using a projection lens. Such image projection apparatuses separate the light into light in the red, green, and blue wavelength ranges, modulate the light of each color using the light modulation unit, and project the light of each color onto the screen so as to overlap the light. Thus, a color image is displayed. In general, a high-pressure mercury vapor lamp that provides high luminous efficiency in the visible light range is used for a light source of the image projection apparatuses. By using a high-pressure mercury vapor lamp, the image projection apparatuses can efficiently emit illumination light.
For example, rear-projection television sets in which an image modulated by a light modulation unit is projected onto the rear surface of a screen have several distinguishing characteristics among the currently available wide-screen television sets. In particular, in recent years, a wide screen liquid crystal television sets and plasma television sets have been in widespread use. The rear projection television sets have an advantage over the liquid crystal television sets and plasma television sets in terms of the weight and cost.
However, it is difficult for the rear-projection television sets to prevent the occurrence of shimmering across the screen known as scintillation caused by the nature of the screen, and therefore, it is difficult to provide images with a natural appearance.
Additionally, in recent years, a laser, in place of the high-pressure mercury vapor lamp, has been in widespread use as a light source for projection. A light source such as a semiconductor laser has a sharp spectrum compared with existing discharge lamps. Accordingly, the laser can provide a wide color range. In addition, the lifetime of the laser is longer than that of the lamp.
However, when the laser is used as the light source, viewers see a number of bright spots across the screen, referred to as speckle. The above-described scintillation is primarily caused by incoherent light from a diffusion material contained in the screen. In contrast, when a laser beam, which is coherent light, is emitted onto the screen, shimmering with perspective occurs due to the interference of the light. At that time, the viewers recognize the shimmering as speckle.
To reduce the speckle, for example, a method for vibrating the entire screen is proposed (refer to, for example, Japanese Unexamined Patent Application Publication No. 55-65940).
In addition, to reduce the speckle and the scintillation, for example, a method for providing two layers having a diffusing function on the screen is proposed (refer to, for example, Japanese Unexamined Patent Application Publication No. 2004-133478).